The present invention relates to a developer used in image forming apparatus, such as electrophotographic apparatus, electrostatic recording apparatus, and magnetic recording apparatus, an image forming method using the developer, and a process-cartridge incorporating the developer. More specifically, the present invention relates to a developer used in image forming apparatus, such as copying machines, printers, facsimile apparatus, and plotters, wherein a toner image is first formed on an image-bearing member and a recording medium such as a transfer(-receiving) material; an image forming method using the developer and the image forming apparatus; and a process-cartridge including the developer.
Hitherto, image forming methods, such as electrophotography, electrostatic recording, magnetic recording, and toner jetting have been known. In the electrophotography, for example, an electrical latent image is formed on a latent image-bearing member which is generally a photosensitive member comprising a photoconductor material by various means, the electrostatic image is developed with a toner to form a visible toner image, and the toner image is, after being transferred onto a recording medium, such as paper, as desired, followed by fixing of the toner image onto the recording medium under application of heat, pressure or heat and pressure to form a fixed image.
Various methods are known, regarding the step of forming a visible image with a toner. For example, as methods for visualizing electrical latent images, there have been known, e.g., the cascade developing method, the pressure developing method, and the magnetic brush developing method using a two-component developer comprising a carrier and a toner. Further, there are also known a non-contact mono-component developing method wherein a toner carried on a toner-carrying member free from contact with a latent image-bearing member is caused to jump onto the latent image-bearing member; a magnetic mono-component developing method wherein a magnetic toner carried on a rotating sleeve containing therein a magnetic field generating means including magnetic poles is caused to jump between the sleeve and a photosensitive member and also a contact mono-component developing method; wherein a toner carried on a toner-carrying member in pressure contact with a latent image-bearing member is transferred under an electric field.
As the developers for visualizing latent images, there are known a two-component(-type) developer comprising a (particulate) carrier and a toner; a mono-component type developer (inclusive of a magnetic toner and a non-magnetic toner) not necessitating a (particulate) carrier. The toner is charged triboelectrically principally owing to friction between the carrier and the toner in the two-component developer, and principally owing to friction between the toner and a charging member, such as a developing sleeve in the mono-component developer.
Further, it has been proposed and widely practiced to use inorganic fine powder as an additive externally added to toner particles in order to improve the flowability or/and triboelectrification characteristic of the toner in both the two-component developer and the mono-component developer.
For example, Japanese Laid-Open Patent Application (JP-A) 5-66608 and JP-A 4-9860 have disclosed a method of adding inorganic fine powder which has been hydrophobized (i.e., hydrophobicity-imparted) and optionally further treated with silicone oil, to toner particles. Further, JP-A 61-249059, JP-A 4-264453 and JP-A 5-346682 have disclosed a method of adding both hydrophobized inorganic fine powder and inorganic fine powder treated with silicone oil.
Further, it has been also proposed to add electroconductive fine powder as an external additive to a developer. For example, it has been widely known to use carbon black as an example of electroconductive fine powder in a form of being attached or stuck onto the surfaces of toner particles, for the purpose of imparting electroconductivity to the toner, or for suppressing an excessive charge of the toner to provide a uniform triboelectric charge distribution. Further, JP-A 57-151952, JP-A 59-168458 and JP-A 60-69660 have disclosed to use electroconductive fine powders, such as tin oxide, zinc oxide and titanium oxide as external additives to high-resistivity magnetic toner particles. JP-A 56-142540 has proposed a developer formed by externally adding electroconductive magnetic particles of, e.g., iron oxide, iron powder or ferrite, to high-resistivity magnetic toner particles so as to satisfy developing performance and transferability by promoting charge induction to the magnetic toner particles with the electroconductive magnetic particles. Further, JP-A 61-275864, JP-A 62-258472, JP-A 61-141452, and JP-A 2-120865 have disclosed the addition of graphite, magnetite, polypyrrole conductor particles and polyaniline conductor particles, respectively, to the toner.
Various methods are also known as methods of forming latent images on image bearing members, such as an electrophotographic photosensitive member and an electrostatic recording dielectric member. In the electrophotography, for example, it is a general practice to uniformly charge a photosensitive member comprising a photoconductor as a latent image-bearing member in a desired polarity and at desired potential, and then subject the photosensitive member to imagewise pattern exposure to form an electrical latent image.
Hitherto, a corona charger (or corona discharger) has been generally used as a charging device for uniformly charging (including a case for charge removal) a latent image-baring member to desired polarity and potential.
A corona charger is a non-contact-type charging device comprising a discharge electrode such as a wire electrode and a shield electrode surrounding the discharge electrode while leaving a discharge opening, and the device is disposed in no contact with an image-bearing member as a member to be charged so that the discharge opening is directed to the image-bearing member for a prescribed charging operation wherein a high voltage is applied between the discharge electrode and the shield electrode to cause a discharge current (corona shower), to which the image-bearing member surface is exposed to be charged to a prescribed potential.
In recent years, a contact charging device has been proposed and commercialized as a charging device for a member to be charged such as a latent image-bearing member because of advantages, such as low ozone-generating characteristic and a lower power consumption, than the corona charging device.
A contact charging device is a device comprising an electroconductive charging member (which may also be called a contact charging member or a contact charger) in the form of a roller (charging roller), a fur brush, a magnetic brush or a blade, disposed in contact with a member-to-be-charged, such as an image-bearing member, so that the contact charging member is supplied with a prescribed charging bias voltage to charge the member-to-be-charged to prescribed polarity and potential.
The charging mechanism (or principle) during the contact charging may include (1) discharge (charging) mechanism and (2) direct injection charging mechanism, and may be classified depending on which of these mechanism is predominant.
(1) Discharge Charging Mechanism in the Contact Charging
This is a mechanism wherein a member is charged by a discharge phenomenon occurring at a minute gap between the member and a contact charging member. As a certain discharge threshold is present, it is necessary to apply to the contact charging member a voltage which is larger than a prescribed potential to be provided to the member-to-be-charged. Some discharge product occurs wile the amount thereof is remarkably less than in a corona charger, and active ions, such as ozone, occur though the amount thereof is small.
(2) Direct Injection Charging Mechanism in the Contact Charging
This is a mechanism wherein a member surface is charged with a charge which is directly injected into the member from a contact charging member. This mechanism may also be called direct charging, injection charging or charge-injection charging. More specifically, a charging member of a medium resistivity is caused to contact a member-to-be-charged to directly inject charges to the member-to-be-charged basically without relying on a discharge phenomenon. Accordingly, a member can be charged to a potential corresponding to an applied voltage to the charging member even if the applied voltage is below a discharge threshold. This mechanism is not accompanied with occurrence of active ions, such as ozone, so that difficulties caused by discharge products can be obviated. However, based on the direct injection charging mechanism, the charging performance is affected by the contactivity of the contact charging member onto the member-to-be-charged. Accordingly, it is preferred that the charging member is provided with a relative moving speed difference from the member-to-be-charged so as to provide a more frequent contact and more dense points of contact with the member-to-be-charged.
As a contact charging device, a roller charging scheme using an electroconductive roller as a contact charging member is preferred because of the stability of charging performance.
During the contact charging according to the conventional roller charging scheme, the above-mentioned discharge charging mechanism (1) is predominant. A charging roller has been formed of a conductive or medium-resistivity rubber or foam material optionally disposed in lamination to provide desired characteristics.
Such a charging roller is provided with elasticity so as to ensure a certain contact with a member-to-be-charged, thus causing a large frictional resistance. The charging roller is moved following the movement of the member-to-be-charged or with a small speed difference with the latter. Accordingly, even if the direct injection charging is intended, the lowering in charging performance, and charging irregularities due to insufficient contact, contact irregularity due to the roller shape and attachment onto the member-to-be-charged, are liable to be caused.
FIG. 3 is a graph illustrating examples of charging efficiencies for charging photosensitive members by several contact charging members. The abscissa represents a bias voltage applied to the contact charging member, and the ordinate represents a resultant charged potential provided to the photosensitive member. The charging performance in the case of roller charging is represented by a line A. Thus, the surface potential of the photosensitive member starts to increase at an applied voltage exceeding a discharge threshold of ca. xe2x88x92500 volts and thereafter increases linearly (at a slope of ca. 1) with respect to the applied voltage. The threshold voltage may be defined as a charging initiation Vth. Accordingly, in order to charge the photosensitive member to a charged potential of xe2x88x92500 volts, for example, it is a general practice to apply a DC voltage of xe2x88x921000 volts, or a DC voltage of xe2x88x92500 volts in superposition of an AC voltage at a peak-to-peak voltage of, e.g., 1200 volts, so as to keep a potential difference exceeding the discharge threshold, thereby causing the charged photosensitive member potential to be converged to a prescribed charged potential.
Thus, in order to obtain a photosensitive member surface potential Vd required for electrophotography, it is necessary to apply a DC voltage of Vd+Vth exceeding the required potential to the charging roller. Such a charging scheme of applying only a DC voltage to a contact charging member may be termed a xe2x80x9cDC charging schemexe2x80x9d.
In the DC charging scheme, however, it has been difficult to charge the photosensitive member to a desired potential, since the resistivity of the contact charging member is liable to change in response to a change in environmental condition, and because of a change in Vth due to a surface layer thickness change caused by abrasion of the photosensitive member.
For this reason, in order to achieve a more uniform charging, it has been proposed to adopt an xe2x80x9cAC charging schemexe2x80x9d wherein a voltage formed by superposing a DC voltage corresponding to a desired Vd with an AC voltage having a peak-to-peak voltage in excess of 2xc3x97Vth is applied to a contact charging member as described in JP-A 63-149669. According to this scheme, the charged potential of the photosensitive member is converged to Vd which is a central value of the superposed AC voltage due to the potential smoothing effect of the AC voltage, whereby the charged potential is not affected by the environmental change.
In the above-described contact charging scheme, the charging mechanism essentially relies on discharge from the contact charging member to the photosensitive member, so that a voltage exceeding a desired photosensitive member surface potential has to be applied to the contact charging member and a small amount of ozone is generated. Further, in the AC-charging scheme for uniform charging, ozone generation is liable to be promoted, a vibration noise (AC charging noise) between the contact charging member and the photosensitive member due to AC voltage electric field is liable to caused, and the photosensitive member surface is liable to be deteriorated due to the discharge.
Fur brush charging is a charging scheme, wherein a member (fur brush charger) comprising a brush of electroconductive fiber is used as a contact charging member, and the conductive fiber brush in contact with the photosensitive member is supplied with a prescribed charging bias voltage to charge the photosensitive member surface to prescribed polarity and potential. In the fur brush charging scheme, the above-mentioned discharge charging mechanism may be predominant.
As the fur brush chargers, a fixed-type charger and a roller-type charger have been commercialized. The fixed-type charger is formed by bonding a pile of medium-resistivity fiber planted to or woven together with a substrate to an electrode. The roller-type charger is formed by winding such a pile about a core metal. A fiber density of ca. 100/mm2 can be relatively easily obtained, but even at such a high fiber density, the contact characteristic is insufficient for realizing sufficiently uniform charging according to the direct injection charging. In order to effect a sufficiently uniform charging according to the direct injection charging, it is necessary to provide a large speed difference between the fur brush charger and the photosensitive member, and this is not practically feasible.
An example of the charging performance according to the fur brush charging scheme under DC voltage application is represented by a line B in FIG. 3. Accordingly, in the cases of fur brush charging using any of the fixed-type charger and the roller-type charger, a high charging bias voltage is applied to cause a discharge phenomenon to effect the charging.
In contrast to the above-mentioned charging schemes, in a magnetic brush scheme, a charging member (magnet brush charger) obtained by constraining electroconductive magnetic particles in the form of a magnetic brush under a magnetic field exerted by a magnet roll is used as a contact charging member, and the magnetic brush in contact with a photosensitive member is supplied with a prescribed charging bias voltage to charge the photosensitive member surface to prescribed polarity and potential. In the magnetic brush charging scheme, the above-mentioned direct injection charging scheme (2) is predominant.
Uniform direct injection charging becomes possible, e.g., by using magnetic particles of 5-50 xcexcm in particle size and providing a sufficient speed difference with the photosensitive member.
An example of the charging performance according to the magnetic brush scheme under DC voltage application is represented by a line C in FIG. 3, thus allowing a charged potential almost proportional to the applied bias voltage.
The magnetic brush charging scheme is however accompanied with difficulties that the device structure is liable to be complicated, and the magnetic particles constituting the magnetic brush are liable to be liberated from the magnetic brush to be attached to the photosensitive member.
Based on the above circumstances, it has been desired to obtain a uniform charging device which is substantially free from discharge products, such as ozone, relies on the direct injection charging mechanism allowing uniform charging at a low applied voltage, is simple and yet can exhibit stable performances.
On the other hand, an image forming method free from generation of waste toner is desired from the viewpoints of economization of resonances, reduction of wastes and effective toner utilization.
The conventional image forming methods have generally included steps of forming a visible image by developing a latent image with a toner, transferring the toner image onto a recording medium such as paper, recovering the residual toner remaining on the latent image-bearing member without being transferred to the recording medium by various cleaning means into a waste toner vessel, and recycling these steps for a subsequent image forming cycle.
The toner recovery or cleaning step has been conventionally performed by using, e.g., a cleaning blade, a cleaning fur brush, a cleaning roller, etc. According to any of these methods, the transfer residual toner is mechanically scraped off or collected by damming into a waste toner vessel. Accompanying increasing demands for resource economization and environmental preservation, it has been desired to construct a system for re-utilizing or disposing the waste toner recovered in the waste toner vessel. In contrast thereto, a so-called toner re-use system of re-cycling the toner recovered in the cleaning step to a developing apparatus for re-use, has been commercialized. The system including such a cleaning step has been generally accompanied with a difficulty that the life of the latent image-bearing member is shortened due to abrasion caused by abutting of the cleaning member against the latent image-bearing member. The provision of the toner re-use system and the cleaning device results in an increase in apparatus size and has provided an obstacle against apparatus compactization.
In contrast thereto, a so-called development and simultaneous cleaning system (developing-cleaning sysetm) or cleanerless system has been proposed as a system free from generation of waste toner. Such a system has been developed principally for obviating image defects, such as positive memory and negative memory due to residual toner. This system has not been satisfactory for various recording media which are expected to receive transferred toner images in view of wide application of electrophotography in recent years.
Cleanerless systems have been disclosed in, e.g., JP-A 59-133573, JP-A 62-203182, JP-A 63-133179, JP-A 64-20587, JP-A 2-302772, JP-A 5-2289, JP-A 5-53482 and JP-A 5-61383. These systems have not been described with desirable image forming methods or toner compositions.
For a developing method suitably applicable to a system essentially free from a cleaning device, a cleanerless system or a development and simultaneous cleaning system, it has been considered essential to rub the electrostatic latent image-bearing member surface with a toner and a toner-carrying member, so that contact developing methods wherein the toner or developer is caused to contact the latent image-bearing member have been principally considered. This is because the mode of rubbing the latent image-bearing member with the toner or developer has been considered advantageous for recovery of the transfer residual toner particles by developing means. However, such a development and simultaneous cleaning system or a cleanerless system is liable to cause toner deterioration, and the deterioration or wearing of the toner-carrying member surface or photosensitive member surface, so that a sufficient solution has not been given to the durability problem. Accordingly, a simultaneous development and cleaning system according to a non-contact developing scheme is desired.
Now, the application of a contact charging scheme to such a development and simultaneous cleaning method or a cleanerless image forming method, is considered. The development and simultaneous cleaning method or the cleanerless image forming method does not use a cleaning member, so that the transfer residual toner particles remaining on the photosensitive member are caused to contact the contact charging system wherein the discharge charging mechanism is predominant. If an insulating toner is attached to or mixed into the contact charging member, the charging performance of the charging member is liable to be lowered.
In the charging scheme wherein the discharge charging mechanism is predominant, the lowering in charging performance is caused remarkably from a time when the toner layer attached to the contact charging member surface provides a level of resistance obstructing a discharge voltage. On the other hand, in the charging scheme wherein the direct injection charging mechanism is predominant, the lowering in charging performance is caused as a lowering in chargeability of the member-to-be-charged due to a lowering in opportunity of contact between the contact charging member surface and the member-to-be-charged due to the attachment or mixing of the transfer residual toner particles into the contact charging member.
The lowering in uniform chargeability of the photosensitive member (member-to-be-charged) results in a lowering in contrast and uniformity of latent image after imagewise exposure, and a lowering in image density and increased fog in the resultant images.
Further, in the development and simultaneous cleaning method or the cleanerless image forming method, it is important to control the charging polarity and charge of the transfer residual toner particles on the photosensitive member and stably recover the transfer residual toner particles in the developing step, thereby preventing the recovered toner from obstructing the developing performance. For this purpose, the control of the charging polarity and the charge of the transfer residual toner particles are effected by the charging member. This is more specifically described with respect to an ordinary laser beam printer as an example. In the case of a reversal development system using a charging member supplied with a negative voltage, a photosensitive member having a negative chargeability and a negatively charged toner, the toner image is transferred onto a recording medium in the transfer step by means of a transfer member applying a positive voltage. In this case, the transfer residual toner particles are caused to have various charges ranging from a positive polarity to a negative polarity depending on the properties (thickness, resistivity, dielectric constant, etc.) of the recording medium and the image area thereon. However, even if the transfer residual toner is caused to have a positive charge in the transfer step, the charge thereof can be uniformized to a negative polarity by the negatively charged charging member for negatively charging the photosensitive member. As a result, in the case of a reversal development scheme, the negatively charged residual toner particles are allowed to remain on the light-part potential where the toner is to be attached, and some irregularity charged toner attached to the dark-part potential is attracted to the toner carrying member due to a developing electric field relationship during the reversal development so that the transfer residual toner at the dark-part potential is not allowed to remain thereat but can be recovered. Thus, by controlling the charging polarity of the transfer residual toner simultaneously with charging of the photosensitive member by means of the charging member, the development and simultaneous cleaning or cleanerless image forming method can be realized.
However, if the transfer residual toner particles are attached to or mixed to the contact charging member in an amount exceeding the toner charge polarity-controlling capacity of the contact charging member, the charging polarity of the transfer residual toner particles cannot be uniformized so that it becomes difficult to recover the toner particles in the developing step. Further, even if the transfer residual toner particles are recovered by a mechanical force of rubbing, they adversely affect the triboelectric chargeability of the toner on the toner-carrying member if the charge of the recovered transfer residual toner particles has not been uniformized. In this way, in the development and simultaneous cleaning or cleanerless image forming method, the continuous image-forming performance and resultant image quality are closely associated with the charge-controllability and attachment-mixing characteristic of the transfer residual toner particles at the time of passing by the charging member.
In order to improve the charge control performance when the transfer residual toner particles are passed by the charging member in the development and simultaneous cleaning method, JP-A 11-15206 has proposed to use a toner comprising toner particles containing specific carbon black and a specific azo iron compound in mixture with inorganic fine powder. Further, it has been also proposed to use a toner having a specified shape factor and an improved transferability to reduce the amount of transfer residual toner particles, thereby improving the performance of the development and simultaneous cleaning image forming method. This image forming method however relies on a contact charging scheme based on the discharge charging scheme and not on the direct injection charging scheme, so that the system is not free from the above-mentioned problems involved in the discharge charging mechanism. Further, these proposals may be effective for suppressing the charging performance of the contact charging member due to transfer residual toner particles but cannot be expected to positively enhance the charging performance.
Further, among commercially available electrophotographic printers, there is a type of development and simultaneous cleaning image forming apparatus including a roller member abutted against the photosensitive member at a position between the transfer step and the charging step so as to supplement or control the performance of recovering transfer residual toner particles in the development step. Such an image forming apparatus may exhibit a good development and simultaneous cleaning performance and remarkably reduce the waste toner amount, but liable to result in an increased production cost and a difficulty against the size reduction.
Further, JP-A 3-103878 discloses to apply powder on a surface of a contact charging member contacting the member-to-be-charged so as to prevent charging irregularity and stabilize the uniform charging performance. This system however adopts an organization of moving a contact charging member (charging roller) following the movement of the member-to-be-charged (photosensitive member) wherein the charging principle generally relies on the discharge charging mechanism simultaneously as in the above-mentioned cases of using a charging roller while the amount of ozone adduct has been remarkably reduced than in the case of using a corona charger, such as scorotron. Particularly, as an AC-superposed DC voltage is used for accomplishing a stable charging uniformity, the amount of ozone adducts is increased thereby. As a result, in the case of a continuous use of the apparatus for a long period, the defect of image flow due to the ozone products is liable to occur. Further, in case where the above organization is adopted in the cleanerless image forming apparatus, the attachment of the powder onto the charging member is obstructed by mixing with-transfer-residual toner particles, thus reducing the uniform charging effect.
Further, JP-A 5-150539 has disclosed an image forming method using a contact charging scheme wherein a developer comprising at least toner particles and electroconductive particles having an average particle size smaller than that of the toner particles is used, in order to prevent the charging obstruction due to accumulation and attachment onto the charging member surface of toner particles and silica fine particles which have not been fully removed by the action of a cleaning blade on continuation of image formation for a long period. The contact charging or proximity charging scheme used in the proposal is one relying on the discharge charging mechanism and not based on the direct injection charging mechanism so that the above-problem accompanying the discharge mechanism accrues. Further, in case where the above organization is applied to a cleanerless image forming apparatus, larger amounts of electroconductive particles and toner particles are caused to pass through the charging step and have to be recovered in the developing step. No consideration on these matters or influence of such particles when such particles are recovered on the developing performance of the developer has been paid in the proposal. Further, in a case where a contact charging scheme relying on the direct injection charging scheme is adopted, the electroconductive fine particles are not supplied in a sufficient quantity to the contact charging member, so that the charging failure is liable to occur due to the influence of the transfer residual toner particles.
Further, in the proximity charging scheme, it is difficult to uniformly charge the photosensitive member in the presence of large amounts of electroconductive fine particles and transfer residual toner particles, thus failing to achieve the effect of removing the pattern of transfer residual toner particles. As a result, the transfer residual toner particles interrupt the imagewise exposure pattern light to cause a toner particle pattern ghost. Further, in the case of instantaneous power failure or paper clogging during image formation, the interior of the image forming apparatus can be remarkably soiled by the developer.
JP-A 10-307456 has disclosed an image forming apparatus adapted to a development and simultaneous cleaning image forming method based on a direct injection charging mechanism and using a developer comprising toner particles and electroconductive charging promoter particles having particle sizes smaller than xc2xd of the toner particle size. According to this proposal, it becomes possible to provide a development and simultaneous cleaning image forming apparatus which is free from generation of discharge product, can remarkably reduce the amount of waste toner and is advantageous for producing inexpensively a small size apparatus. By using the apparatus, it is possible to provide good images free from defects accompanying charging failure, and interruption or scattering of imagewise exposure light. However, a further improvement is desired.
Further, JP-A 10-307421 has disclosed an image forming apparatus adapted to a development and simultaneous cleaning method, based on the direct injection charging mechanism and using a developer containing electroconductive particles having sizes in a range of {fraction (1/50)}-xc2xd of the toner particle size so as to improve the transfer performance.
JP-A 10-307455 discloses the use of electroconductive fine particles having a particle size of 10 nm-50 xcexcm so as to reduce the particle size to below one pixel size and obtain a better charging uniformity.
JP-A 10-307457 describes the use of electroconductive particles of at most about 5xcexc, preferably 20 nm-5 xcexcm, so as to bring a part of charging failure to a visually less recognizable state in view of visual characteristic of human eyes.
JP-A 10-307458 describes the use of electroconductive fine powder having a particle size smaller than the toner particle size so as to prevent the obstruction of toner development and the leakage of the developing bias voltage via the electroconductive fine powder, thereby removing image defects. It is also disclosed that by setting the particle size of the electroconductive fine powder to be larger than 0.1 xcexcm, the interruption of exposure light by the electroconductive fine powder embedded at the surface of the image-bearing member is prevented to realize excellent image formation by a development and simultaneous cleaning method based on the direct injection charging scheme. However, a further improvement is desired.
JP-A 10-37456 has disclosed a development and simultaneous cleaning image forming apparatus capable of forming without causing charging failure or interruption of imagewise exposure light, wherein electroconductive fine powder is externally added to a toner so that the electroconductive powder is attached to the image-bearing member during the developing step and allowed to remain on the image-bearing member even after the transfer step to be present at a part of contact between a flexible contact charging member and the image-bearing member.
These proposals however have left a room for further improvement regarding the stability of performance during repetitive use for a long period and performance in the case of using smaller size toner particles in order to provide an enhanced resolution.
The use of electroconductive particles having a specified average particle size externally added to toner particles has been proposed. For example, JP-A 9-146293 has proposed a toner comprising fine powder A having an average particle size of 5-50 nm and fine powder B having an average particle size of 0.1-3 xcexcm externally added to and attached to toner particles at a strength larger than specified so as to reduce the proportion of the powder B isolated from the toner particles. Further, JP-A 11-95479 has proposed a toner containing hydrophobized inorganic oxide and electroconductive silica particles having specified particle sizes, but the electroconductive silica particles are added to merely promote the leakage of charge excessively accumulated at the toner.
Further, not a few proposals have been made regarding toner having specific particle size distributions and shapes. A proposal of a toner having a particle size distribution and a circularity measured by a flow-type particle image analyzer has been proposed in recent years JP-A 9-197714. As for proposals of toners having specified particle size distributions and shapes taking account of contributions of external additives, JP-A 11-174731 has proposed a toner containing inorganic fine powder A having a specific circularity and an average longer-axis diameter of 10-400 nm and non-spherical inorganic fine powder B wherein the powder B is expected to function as a spacer for suppressing the inorganic fine powder A from being embedded at the surface of the toner mother particles. JP-A 11-202557 has also proposed a toner having specific particle size distribution and circularity so as to provide a developed toner image having an increased density, thereby suppressing the image tailing phenomenon, and to improve the preservability of the toner in a high temperature/high humidity environment.
JP-A 11-194530 has proposed a toner containing externally added fine particles A of 0.6-4 xcexcm and inorganic fine powder B and having a specific particle size distribution, wherein the toner deterioration due to embedding of the inorganic fine powder B at the toner particle surface is suppressed by the presence of the externally added fine particles A, and the attachment to or liberation from the toner particles of the externally added fine particles A is not considered. JP-A 10-83096 has proposed a toner comprising electroconductive fine particles and silica fine particles externally added to spherical resin fine particles enclosing a colorant therein, wherein the toner particles are expected to have a surface electroconductivity, thereby accelerating the movement and exchange of carrier between the toner particles and enhancing the toner triboelectric charge uniformity.
As described above, sufficient consideration has not been paid to external additives for a developer used in the image forming method including a direct injection charging step, or the development and simultaneous cleaning image forming method or cleanerless image forming method, and therefor a developer containing external additives fully adapted to such image forming methods has not been proposed.
In view of the above-mentioned problems of prior art, an object of the present invention is to provide a developer capable of toner image formation through a satisfactory developing-cleaning step (i.e., a developing and simultaneous cleaning step).
Another object of the present invention is to provide a developer allowing a simple and stable charging operation based on the direct injection charging mechanism substantially free from generation of discharge products such as ozone and allowing uniform charging at a low applied voltage.
Another object of the present invention is to provide an image forming method allowing a developing-cleaning step which can remarkably reduce the amount of waste toner and is advantageous for providing an inexpensive and small-sized image forming apparatus.
Another object of the present invention is to provide an image forming method including a charging step based on the direct injection charging mechanism substantially free from generation of discharge products such as ozone and allowing uniform charging at a low applied voltage, whereby a stable charging can be performed conveniently and without causing charging failure even in repetitive operation for a long period.
Another object of the present invention is to provide an image forming method adapted to a cleanerless image forming mode not requiring an independent cleaning step while ensuring a good and stable charging performance, and a process-cartridge therefor.
Another object of the present invention is to provide an image forming method adapted to a developing-cleaning step allowing excellent performance in recovery of transfer residual toner particles, and a process-cartridge therefor.
A further object of the present invention is to provide an image forming method including a developing-cleaning step allowing stable formation of good images even when toner particles of smaller particle size are used for providing a higher resolution, and a process-cartridge therefor.
According to the present invention, there is provided a developer for developing an electrostatic latent image, including: toner particles each comprising a binder resin and a colorant, inorganic fine powder having a number-average particle size of 4-80 nm based on primary particles, and electroconductive fine powder; wherein the developer has a number-basis particle size distribution in the range of 0.60-159.21 xcexcm including 15-60% by number of particles in the range of 1.00-2.00 xcexcm, and 15-70% by number of particles in the range of 3.00-8.96 xcexcm, each particle size range including its lower limit and excluding its upper limit.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.